This invention relates generally to solder bonding techniques for integrated circuit devices.
Referring to FIG. 10, commonly solder is deposited on a solder pad 62 which is coupled to other electrical components on an integrated circuit by a trace 60. The solder deposition area is defined by the inwardmost edge 64 of a solder mask. Thus, in the embodiment illustrated in FIG. 10, the solder is deposited inside the circle 64. The solder mask prevents solder outflow over the mask thereby preventing the solder from moving outwardly beyond the edge 64.
The solder may be in the form of conventional solder balls which are deposited in a solid configuration and then reflowed thereafter. Alternating the solder may be a liquid or paste upon deposition.
Referring to FIG. 11, one problem with existing techniques for depositing solder is that when soft the solder 66 tends to wick along the trace 60. Without limitation, it is believed that the wicking is a result of capillary attraction between the solder 66 and the trace 60. As a result, the solder 66 ends up being displaced with respect to the pad 62, as indicated in FIG. 11. In particular, the solder may abut the solder mask edge 64. Generally, the solder does not extend onto the solder mask since the mask functions to control solder flow.
Thus, improper contact may result between the solder and the solder pad 62 as a result of the wicking action of the solder. Of course, this problem may be reduced by decreasing the diameter of the opening 64 in the solder mask. However, this creates tighter tolerances in the process flow. One adverse result may be that the solder mask opening is misaligned to the pad 62 to such an extent that the solder mask opening does not permit the solder to be placed on the pad.
In ball grid array (BGA) packaging techniques an array of solder pads may be aligned with an array of solder balls. If the balls tend to wick away from their solder pads, the balls may become misaligned with other balls in the array. Thus, there may be no way to cause an integrated circuit connector to appropriately connect to all the balls because all the balls have been randomly misaligned. Referring to FIG. 12, the ball 68 on the top has wicked to the right because its trace 60 extends to the right whereas the ball 76 on the bottom has wicked to the left because of the leftward extension of its trace 70. The center line “CL” of the pads 62 and 72 may have been the projected alignment between the balls. In fact the balls are substantially misaligned.
Still another problem that may arise in the prior art is the surface action effects of the edge of the solder resist mask. FIG. 13 illustrates a conventional solder mask defined pad (SDP). In this case, the useful portion of the pad 80 is effectively defined by the opening 82 in the solder mask. This is because the size of the opening 82 is less than the size of the pad 80. Thus, wicking along the trace 78 may be prevented. However, the mask may tend to attract the solder 84 to its edge, for example as a result of surface attraction effects. Again, the problem is similar to the problem described previously in that the solder tends to be attracted away from its desired location.
Thus, there is a need for better ways to appropriately position solder on bond pads coupled to conductive traces.